The present invention relates generally to permanent magnet generators, and more particularly to methods and systems for regulating voltage in permanent magnet generators.
Rotary generators convert mechanical torque into electrical power by inducing stator voltages using a magnetized rotor, or inducing rotor voltages using a magnetized stator. In either case, the magnetized element moves relative to a plurality of windings, producing a varying magnetic flux through those windings which induces a current and voltage according to Faraday's Law. For the purposes of this discussion, a magnetized rotor and a stator with a plurality of induction windings will be assumed, although one skilled in the art will recognize that this configuration can be reversed.
Permanent magnet generators provide magnetic fields with permanent magnets such as rare earth magnets, and have numerous advantages over wound field rotor synchronous generators. As long as a permanent magnet rotor rotates, it produces a varying magnetic field. Whenever this magnetic field passes through stator windings of the permanent magnet generator, the resulting changing magnetic flux induces currents and voltages on the windings. This simplicity and reliability is useful in a variety of applications, but makes de-energizing a permanent magnet generator difficult. Stator faults and changes in power requirements can necessitate rapid changes to induced voltage. Stator faults between windings or phases of a rotary generator, in particular, can cause rapid component deterioration if voltages are not immediately curtailed upon fault detection. In wound field generators, the field excitation voltage can be controlled by rapidly demagnetizing the rotor. In permanent magnet generators, alternative methods are needed, since a permanent magnet rotor cannot be demagnetized, and mechanically halting rotation may be infeasible or slow.
A variety of techniques have been developed for this purpose to cancel of divert magnetic flux away from stator windings. Some permanent magnet generators utilize mechanical bridges which can be closed to provide alternative flux paths. Others provide persistent flux paths which are magnetically saturated during ordinary generator operation, but which are desaturated to divert flux away from stator windings during fault conditions. Still other approaches rely on control windings to produce countervailing flux which to cancel net flux through stator windings. Although a multitude of approaches have been proposed to de-energize permanent magnet generators, no single approach dominates the field.